If worse comes to worse - drill through the frame and bolt them in place. In my climate, I'd get #8 stainless, but plated might well be fine in yours. I wouldn't got too large - the pressure is not on the bolt, it's just holding the seat belt material to the frame, but if the larger the holes you drill, the more you would be weakening the metal frame. Get what are called "fender washers" - just washer with more "outside" compared to the "hole". And get some sort of locking nut rather than regular nut also.
Debbie Ann wrote:Craig, That sounds like a very good idea too!! If I can figure out a good way to attach them they would be super sturdy, very economical and colorful. They would fit right in to my funky and eclectic gardens!